A. Technical Field
The present invention relates to hearing devices, and, more particularly, to miniature hearing devices that are deeply positioned in the ear canal for improved energy efficiency, sound fidelity, and inconspicuous wear.
B. Description of the Prior Art
Brief Description of Ear Canal Anatomy
The external acoustic meatus (ear canal) is generally narrow and tortuous as shown in the coronal view in FIG. 1. The ear canal 10 is approximately 25 mm in length from the canal aperture 17 to the tympanic membrane 18 (eardrum). The lateral (away from the tympanic membrane) part, a cartilaginous region 11, is relatively soft due to the underlying cartilaginous tissue. The cartilaginous region 11 of the ear canal 10 deforms and moves in response to the mandibular (jaw) motions, which occur during talking, yawning, eating, etc. The medial (towards the tympanic membrane) part, a bony region 13 proximal to the tympanic membrane, is rigid due to the underlying bony tissue. The skin 14 in the bony region 13 is thin (relative to the skin 16 in the cartilaginous region) and is more sensitive to touch or pressure. There is a characteristic bend 15 that roughly occurs at the bony-cartilaginous junction 19, which separates the cartilaginous 11 and the bony 13 regions. The magnitude of this bend varies significantly among individuals. The internal volume of the ear canal between the aperture 17 and tympanic membrane is approximately 1 cubic centimeter (cc).
A cross-sectional view of the typical ear canal 10 (FIG. 2) reveals generally an oval shape and pointed inferiorly (lower side). The long diameter (DL) is along the vertical axis and the short diameter (DS) is along the horizontal axis. Canal dimensions vary significantly among individuals as shown below in the section titled Experiment A.
Physiological debris 4 in the ear canal is primarily produced in the cartilaginous region 11, and includes cerumen (earwax), sweat, decayed hair, and oils produced by the various glands underneath the skin in the cartilaginous region. There is no cerumen production or hair in the bony part of the ear canal. The ear canal 10 terminates medially with the tympanic membrane 18. Laterally and external to the ear canal is the concha cavity 2 and the auricle 3, both also cartilaginous.
Several types of hearing losses affect millions of individuals. Hearing loss particularly occurs at higher frequencies (4000 Hz and above) and increasingly spreads to lower frequencies with age.
The Limitations of Conventional Canal Hearing Devices.
Conventional hearing devices that fit in the ear of individuals generally fall into one of 4 categories as classified by the hearing aid industry: (1) Behind-The-Ear (BTE) type which is worn behind the ear and is attached to an ear mold which fits mostly in the concha; (2) In-The-Ear (ITE) type which fits largely in the auricle and concha cavity areas, extending minimally into the ear canal; (3) In-The-canal (ITC) type which fits largely in the concha cavity and extends into the ear canal (see Valente M., Strategies for Selecting and Verifying Hearing Aid Fittings. Thieme Medical Publishing. pp. 255-256, 1994), and; (4) Completely-In-the-Canal (CIC) type which fits completely within the ear canal past the aperture (see Chasin, M. CIC Handbook, Singular Publishing (“Chasin”), p. 5, 1997).
The continuous trend for the miniaturization of hearing aids is fueled by the demand for invisible hearing products in order to alleviate the social stigma associating hearing loss with aging and disability. In addition to the cosmetic advantage of canal devices (ITC and CIC devices are collectively referred to herein as canal devices), there are actual acoustic benefits resulting from the deep placement of the device within the ear canal. These benefits include improved high frequency response, less distortion, reduction of feedback and improved telephone use (Chasin, pp. 10-11).
However, even with these significant advances leading to the advent of canal devices, there remains a number of fundamental limitations associated with the underlying design and configurations of conventional canal device technology. These problems include: (1) oscillatory (acoustic) feedback, (2) custom manufacturing and impression taking, (3) discomfort, (4) occlusion effect and, (5) earwax. These limitations are discussed in more detail below.                (1) Oscillatory feedback occurs when leakage (arrows 32 and 32′ in FIG. 3) from sound output 30, typically from a receiver 21 (speaker), occur via a leakage path or a vent 23. The leakage (32′) reaches a microphone 22 of a canal hearing device 20 causing sustained oscillation. This oscillatory feedback is manifested by “whistling” or “squealing” and is not only annoying to hearing aid users but also interferes with their communication. Oscillatory feedback is typically alleviated by tightly occluding (sealing) the ear canal. However, due to imperfections in the custom manufacturing process (discussed below) or to the intentional venting incorporated within the hearing device (also discussed below) it is often difficult if not impossible to achieve the desired sealing effect, particularly for the severely impaired who require high levels of amplification. Oscillatory feedback primarily typically occurs at high frequencies due to the presence of increased gain at these frequencies.        (2) Custom manufacturing and impression taking: Conventional canal devices are custom made according to an impression taken from the ear of the individual. The device housing 25 (FIG. 3), known as shell, is custom fabricated according to the impression to accurately assume the shape of the individual ear canal. Customizing a conventional canal device is required in order to minimize leakage gaps, which cause feedback, and also to improve the comfort of wear. Custom manufacturing is an imperfect process, time consuming and results in considerable cost overheads for the manufacturer and ultimately the hearing aid consumer (user). Furthermore, the impression taking process itself is often uncomfortable for the user.        (3) Discomfort, irritation and even pain frequently occur due to canal abrasion caused by the rigid plastic housing 25 of conventional canal devices 20. This is particularly common for canal devices that make contact with the bony region of the ear canal. Due to the resultant discomfort and abrasion, hearing devices are frequently returned to the manufacture in order to improve the custom fit and comfort (Chasin, p. 44). “The long term effects of the hearing aid are generally known, and consist of atrophy of the skin and a gradual remodeling of the bony canal. Chronic pressure on the skin lining the ear canal causes a thinning of this layer, possibly with some loss of skin appendages” (Chasin, p. 58).        (4) The occlusion effect is a common acoustic problem caused by the occluding hearing device. It is manifested by the perception of a person's “self-sounds” (talking, chewing, yawning, clothes rustling, etc) being loud and unnatural compared to the same sounds with the open (unoccluded) ear canal. The occlusion effect is primarily due to the low frequency components of self-sounds and may be experienced by plugging the ears with fingers while talking for example. The occlusion effect is generally related to sounds resonating within the ear canal when occluded by the hearing device. The occlusion effect is demonstrated in FIG. 3 when “self-sounds” 35, emanating from various anatomical structures around the ear (not shown), reach the ear canal 10. When the ear canal is occluded, a large portion of self-sounds 35 are directed towards the tympanic membrane 18 as shown by arrow 34. The magnitude of “occlusion sounds” 34 can be reduced by incorporating an “occlusion-relief vent” 23 across the canal device 30. The occlusion-relief vent 23 allows a portion of the “occlusion sounds” 35 to leak outside the ear canal as shown by arrow 35′.        The occlusion effect is inversely proportional to the residual volume of air between the occluding hearing device and the tympanic membrane. Therefore, the occlusion effect is considerably alleviated by deeper placement of the device in the ear canal. However, deeper placement of conventional devices with rigid enclosures is often not possible for reasons including discomfort as described above. For many hearing aid users, the occlusion effect is not only annoying, but is often intolerable leading to discontinued use of the canal device.        (5) Earwax build up on the receiver of the hearing device causing malfunction is well known and is probably the most common factor leading to hearing aid damage and repair (Oliveira, et al, The Wax Problem: Two New Approaches, The Hearing journal, Vol. 46, No. 8).        
The above limitations in conventional canal devices are highly interrelated. For example, when a canal device is worn in the ear canal, movements in the cartilaginous region “can lead to slit leaks that lead to feedback, discomfort, the occlusion effect, and ‘pushing’ of the aid from the ear” (Chasin, pp. 12-14). The relationship between these limitations is often adverse. For example, occluding the ear canal tightly is desired on one hand to prevent feedback. However, tight occlusion leads to the occlusion effect described above. Attempting to alleviate the occlusion effect by a vent 23 provides an opportunistic pathway for output sound 30 (FIG. 3) to leak back (arrows 32 and 32′) and cause feedback. For this reason alone, the vent 23 diameter is typically limited in CIC devices to 0.6-0.8 mm (Chasin, pp. 27-28).
Review of State-of the-Art in Related Hearing Device Technology
Ahlberg, et al and Oliviera, et al in U.S. Pat. Nos. 4,880,076 and 5,002,151 respectively, disclose an earpiece with sound conduction tube having a solid compressible polymeric foam assembly. The retarded recovery foam must first be compressed prior to its insertion into the ear canal to recover and seal within. However, a compressible polymeric foam can be uncomfortable and irritating to the ear canal after recovering (i.e., being decompressed). Furthermore, many impaired individuals do not possess the required manual dexterity to properly compress the foam prior to insertion in the ear canal.
Sauer et al., in U.S. Pat. No. 5,654,530, disclose an insert associated with an ITE device (FIG. 1 in Sauer) or a BTE device (FIG. 2 in Sauer). The insert is a “sealing and mounting element” for a hearing device positioned concentrically within the insert. Sauer's disclosure teaches an insert for ITEs and BTEs; it does not appear to be concerned with inconspicuous hearing devices that are deeply or completely inserted in the ear canal, or with delivering sound and sealing in the bony region of the canal.
Garcia et al., in U.S. Pat. No. 5,742,692 disclose a hearing device (10 in FIG. 1 of Garcia) attached to a flexible seal (collar 30) which is fitted in the bony region of the ear canal. The device 10 is substantially positioned in the cartilaginous region along with the collar 30, which is partially positioned over the housing. It is not clear how the disclosed device with its contiguous housings and seal configuration can fit comfortably and deeply in many small and contoured canals.
Voroba et al in U.S. Pat. No. 4,870,688 discloses a mass-producible hearing aid comprising a solid shell core (20 in FIGS. 1 and 2 of Veroba) which has a flexible covering 30 affixed to the exterior of the rigid core 20. The disclosed device further incorporates a soft resilient bulbous tubular segment 38 for delivering sound closer to the tympanic membrane and sealing within. Similarly, it is unlikely for this contiguous device/tubular segment to fit comfortably and deeply in many small and contoured canals.
None of above inventions addresses the occlusion effect other than by the conventional vent means, which are known to adversely cause oscillatory feedback.
McCarrell, et al, Martin, R., Geib, et al., Adelman R., and Shennib, et al., in U.S. Pat. Nos. 3,061,689, RE 26,258, 3,414,685, 5,390,254, and 5,701,348, respectively, disclose miniature hearing devices with a receiver portion flexibly connected to a main part. Along with various accessories including removable acoustic seals, these devices have the advantage of fitting a variety of ear canal sizes and shapes thus are mass-producible in principle. However, the flexible or articulated receiver portion in these devices requires flexible mechanical and electrical connections, which result in added cost and reduced reliability compared with conventional devices which comprise instead immobile receivers contained in a singular rigid housing. Furthermore, by incorporating a seal mechanism concentrically over a rigid receiver, or a rigid receiver section, the compressibility of the seal, regardless of its compliance, is severely limited by the rigid core section which has a substantial diameter compared with the ear canal.
Ward et al., in U.S. Pat. Nos. 5,031,219 and 5,201,007, disclose a sound conduction tube (60 in Ward) for conveying amplified sound to the ear canal within the bony region in close proximity to the tympanic membrane (30). The invention also comprises a “flexible flanged tip” (70), essentially a seal, for acoustically sealing in the bony region. Ward et al. state two main objectives, viz.: “To assure proper operation of the present invention, the hearing aid should [1] neither prevent unamplified sound received at the ear from entering the ear canal, [2] nor should it contact a substantial portion of the skin lining the ear canal” (lines 32-36 col. 4 in the '219 patent and lines 37-41 col. 4 in the '007 patent). The present applicants have concluded that these limitations cause serious disadvantages for practical implementation in canal hearing devices. First, unamplified sound is allowed to freely enter the ear canal which also allows amplified sound in the bony region, which partially leaks into the cartilaginous region, to feed back to the microphone of the device and cause oscillatory feedback. This occurs because some level of leakage is always present through any acoustic barrier. Second, the contact area of the seal with the ear canal is minimized (see FIGS. 1 and 5A-5F in '219 and '007, and the recital “it has been found that a suitable edge 72 thickness is approximately 0.05 to 2 millimeters.”), so that adequate sealing along this small contact area is not possible without exerting considerable pressure on the ear canal. This is particularly problematic for canal devices having a microphone relatively in close proximity to leakage in the open ear canal as suggested and shown in the figures.
Although Ward et al. briefly mention potential applications of their devices for canal devices (lines 22-26 col. 4 in '219 and lines 27-31 col. 4 in '007), the practical application is limited to BTE hearing aids with microphones far and away external to the ear canal (91 in FIG. 3. in both the '219 and '007 patents).
It is a principal objective of the present invention to provide a highly inconspicuous hearing device.
A further objective is to provide a hearing device which comfortably delivers amplified sound in the bony region in close proximity to the tympanic membrane.
Another objective is to provide an acoustic system in which acoustic sealing is maximized for prevention of feedback while simultaneously minimizing occlusion effects.
Still another objective is to improve the frequency response of delivered sound, particularly at higher frequencies while reducing occlusion sounds particularly at lower frequencies.
Yet another objective is to provide a mass-producible hearing device design which does not require custom manufacturing or individual ear canal impression.
Unlike the prior art, the present invention is not concerned with allowing external unamplified sounds to enter the ear canal.